The invention relates to a method and apparatus for measuring the rheological properties of an extrudable material.
Rheological properties of a material are those which relate to the flow of the material, and are of importance in processing rubber and plastic materials. For example, in forming rubber or plastic articles, the articles are shaped by injection molding, transfer molding, compression molding, extrusion, and other processes, wherein a mass of flowable material is subjected to differential pressures so as to change its shape. The behavior of the material under different conditions of pressure, temperature and shear will determine the process equipment and techniques required to form the final article. Also, since the properties of the flowable material may vary from batch to batch it is desirable to be able to predict the rheological behavior of a batch before it is used in the production process.
A number of methods have been used to predict the rheological behavior of rubber or plastic materials, with varying degrees of success. The Mooney viscometer is widely used to measure the viscosity of natural and synthetic rubber polymers. This device employs a fixed stator and movable rotor with a small sample of rubber between them. The measure of the force required to move the rotor and shear the rubber sample is a measure of the viscosity of the rubber.
An improved device employing a conical disc imbedded in a rubber sample and oscillating over a small arc has given more insight into the rubber properties.
More recently, the Capillary Rheometer has been introduced, which extrudes a sample of polymer through an orifice. Stress-strain measurements at different shear rates and temperatures provide insight into the rheological behavior of the polymer at higher shear rates. As a further adjunct to the Capillary Rheometer, the diameter of the polymer extrudate has been optically measured, to yield die swell data.
Despite the various known methods for evaluating the rheological behavior of rubber and plastic materials a need exists for a method and apparatus which can quickly and accurately produce a wide variety of rheological data from a small sample of the material, and provide predictions of the processing behavior for all steps of production.
Variations in "raw", or uncompounded polymer from batch to batch can result from differences in the macrostructure of the polymer, such as its molecular weight distribution, degree of branching, and content of "gel" (less soluble portions). These variations can result in processing differences throughout the entire production cycle.
When compounding ingredients, such as vulcanizing or cross-linking agents, antidegradants, fillers and plasticizers are added to a rubber or plastic material, its rheological behavior is changed. Inadequate mixing of these materials, especially fillers, can cause heterogeneity, which often will negatively affect the processing of the compounded material.
In the case of plastic or rubber materials which are cross-linkable (i.e., thermosetting) another potential processing problem is introduced when the cross-linking agent is added. If the cross-linking process begins prematurely, the probability of processing difficulties is greatly multiplied. Since it is usually desirable to have cross-linking occur as rapidly as possible once the material assumes its final form, the cross-linking rate must be carefully controlled at a rate neither too slow nor too fast. In rubber compounding the effect of cross-linking on processing is known as "scorch". Known methods for evaluating the scorch time of a compound yield results indicative of the onset of cross-linking as it is evidenced in the bulk of the material, by a viscosity increase. Small localized scorch, sometimes termed "micro-scorch" may occur because of imperfect dispersion, and usually happens before the bulk scorch effects are noticed. A measure of the incidence and severity of micro-scorch not obtainable by known scorch tests would be of great value to rubber processers.
In summary, the need exists for a method and apparatus whereby the rheological properties of a rubber or plastic material in all stages of its processing could be accurately, easily and quickly obtained by testing a small sample of the material.